PROCEEDINGS OF THE ACADEMY OF [NoV., 



ORDOVICIAN BASALTS AND QUARTZ DIABASES IN LEBANON COUNTY, 



PENNSYLVANIA' 



SAMUEL G. GORDON. 



As no Paleozoic volcanic rocks have hitherto been discovered in 

 Pennsylvania, the occurrence of an Ordovician basalt flow in Leb- 

 anon County is of interest, especially in view of the fact that deep 

 seated intrusives occur in the Octoraro schists of similar age to the 

 southeast. 2 



The basalts and quartz diabases described below occur in the 

 Lebanon quadrangle, just, south of the Swatara Creek, between 

 Jonestown and Lebanon (see Fig. 1). The area is underlain by a 

 thick series of Martinsburg shales, whose outcrop has been con- 

 siderably extended in width by folding and faulting. Intrusive in 

 it are sills and dikes of quartz diabase, to the northwest of which 

 lie the interbedded basalt flows. 



Martinsburg Formation. The Martinsburg formation consists of 

 a thick series of gray, greenish, and reddish shales, with interbedded 

 sandstones, and thin beds of dolomite,^ — ^the last notably in the 

 vicinity of the Swatara. A white sandstone is exposed on the 

 Bunker Hills. Slaty cleavage has been developed in the shales. 

 The formation has been overturned, dipping steeply toward the 

 southeast throughout this area. 



In the vicinity of the intrusive diabases, the shales have been 

 metamorphosed to dense dark grayish, greenish, or reddish rocks 

 containing veins of epidote or vesuvianite. Such rocks are well 

 exposed just south of the Swatara, 2 miles southeast of Jonestown, 

 and one half mile east of Bunker Hill Station in a cut on the Jones- 

 town-Lebanon road. Under the microscope the rocks are seen to 

 be aggregates of orthoclase, tremolite, epidote, vesuvianite, chlorite, 

 quartz, and rounded zircons. 



^ The district was visited during the latter part of August, 1920. The writer 

 is indebted to Mr. Frank J. Keeley for the privilege of examining his sections of 

 other Pennsylvania diabases, and to Dr. Edgar T. Wherry for a critical exami- 

 nation of this paper. 



^ Basic breccia (ouachitite) and dikes of nepheline syenite, leucite tinquaite, 

 and camptonite of Post-ordovician age occur in the northwestern corner of the 

 FrankUn Furnace quadrangle, New Jersey. U. S. G. S. Franklin Furnace 

 Folio, 162, 1908. 



